Wavelength division multiplexing (WDM) is a technique used to transmit multiple channels of data simultaneously over the same optic fiber. At a transmitter end, data channels having different wavelengths or, colors if you will, are combined, or multiplexed together into a single fiber. The fiber can simultaneously carry multiple channels in this manner. Before being multiplexed into a single fiber, each one of these data channel requires its own laser which must be coupled to an individual fiber. At a receiving end, these multiplexed channels are separated prior to demodulation using appropriate wavelength filtering techniques.
The need to transmit greater amounts of data over a fiber has led to so-called Dense Wavelength Division Multiplexing (DWDM). DWDM involves packing additional channels into a given wavelength range. The resultant narrower spacing between adjacent channels in DWDM systems demands precision wavelength accuracy from the transmitting laser diodes. Further, DWDM as well as single channel systems require tight mechanical tolerances and precision coupling between various components.
One of the major challenges in the optoelectronic assembly process is to couple light from one chip to another chip or waveguide. The light or optical signal must be focused through an optical system onto a receiving waveguide a few microns wide. Placement accuracy of the optical components required to achieve acceptable coupling are often in the sub-micron range. Active components such as laser diode, optical amplifier, photodiode and optical modulator, usually need to be rigidly mounted to a substrate to insure proper heat dissipation or provide acceptable electrical path to maintain integrity of the high frequency signal. Typical assembly process of a laser transmitter used in the opto-electronic communication field can be as follow:
First, the laser diode is mounted onto a substrate at a predefined position. The substrate will provide an effective thermal path to evacuate the heat load generated by the diode thus keeping it within operating temperature range. The placement accuracy of the diode will vary in function of the design requirements but suffice to say that the less stringent the accuracy requirement is, the easier and cheaper the assembly process will be.
The second step will be to attach the second component on the substrate. This component could be an optical fiber or a lens to collimate or focus the laser beam onto a receiving waveguide such as an optical fiber. Innacuracy in the laser diode position can be compensated by adjusting the position of this second component. The second components is then bonded or otherwise secured to a surface while being careful to keep the alignment.
Additional components can be further added to the assembly by repeating the second step. Each one of these steps can be done either passively; use some type of external feedback such as machine vision to adjust their positions relative to the previous components or; be done actively in which case the laser diode is powered and the signal strength and/or signal integrity is monitored during the alignment process.
Furthermore, the assembly needs to be reliable. That is, the finished assembly including the bonding must be stable under temperature cycling, aging, shock, vibration, and any other condition that the assembly may reasonably be expected to encounter. To further complicate matters, most assemblies include more than just two components which must all be aligned. Each additional component further adds to the challenge. It is very difficult to hold the alignment while making the bond. Often some shift or movement occurs between the components which, if greater than the maximum tolerances dictate, may render the component unworkable or at least seriously degrade performance.
For clarity a reference frame is defined where the z axis is parallel to the apparatus optical axis. The X axis is normal to the optical axis (z) and in the horizontal plane and finally the Y axis is normal to the plane formed by the X and Z.axes.
Various fiber optic support devices have been devised to facilitate fiber optic alignment. For example, U.S. Pat. No. 5,619,609 by Pan discloses a clip for supporting an end of an optical fiber relative to a mount surface. A sleeve is disposed over the optical fiber adjacent to its end. The clip comprises a clip body with an upper and lower surface, with a flange disposed adjacent to the lower surface. The flange is affixable to the mount surface, and walls extend from the upper surface of the body to define a channel at which the clip is affixable about the sleeve. When the sleeve is affixed within the channel, the body rigidly couples the sleeve to the flange, thereby avoiding misalignment between the optical fiber and any optical device which is on or supported by the mount surface. It appears that y-direction alignment of the fiber may be accomplished by manipulation of the sleeve up and down within the clip and x direction alignment may be accomplished by sliding the clip relative to the mount surface. Finally the z alignment is accomplished by either sliding the ferrule in the clip or by sliding the clip on the mount surface along the optical axis. Once aligned, the clip may be secured to the mounting surface and the sleeve to the clip such as by soldering or laser spot welding.
While the above method may be advantageous, it requires handling two different components and also requires a two step alignment and attachment process.